ATZ uses the Chain Reaction rule system, which is free to download. If you want to see what it's like before purchase. The system allows you to play solo, coop, or head to head.
The ATZ rule set is fairly well written, but could use a little reorganization. However reading it through once and then sitting down with some miniatures to move around allowed me to figure it out with no issues. Two Hour Wargames also has a excellent Yahoo support group, if you have questions. The system is completely different than any other miniatures system I've played, so it may take some getting used to, but it really is great fun solo and coop. It has light rpg elements as well.

The ATZ game itself is excellent. It may be the best set of zombie plague rules out there. It plays fast and has lots of pop up surprises. I am using 15 mm figures from Rebel Minis, which are awesome in detail and price. My terrain is from World Works, and I printed it to 66% size which worked really nice. You can also use model train stuff which is very close to 15mm, can't remember the letter sacle at the moment. The game works with any scale miniatures, I just chose 15mm because it is much cheaper. Although there are many awesome paper miniatures on this site, as well great metal/plastic miniatures for zombies by a bunch of different manufacturers.

If you like zombie games and want to have a good time I highly recommend this rule set.